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Old 11-02-2013, 10:31   #16
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Re: Settle an argument

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The minute you start talking about a partial vacuum, you've entered the rhelm of physics. It doesn't take a great intellectual leap to go from being a creationist or a climate science denier to claiming that a flat sail would work aerodynamically.

The real question isn't science--it's engineering: just how are you going to keep a flat sail flat?
A winged sail would be a good comparison.
These are not flat, but are a symmetrical aerofoil, which has equal camber on both sides, like flat sheet.

You could build a winged sail out of a flat sheet, I am not sure why you want too, but sometimes in settling an argument reason does not play a role
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Old 11-02-2013, 10:31   #17
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Re: Settle an argument

Is your FIL a member of the Flat Earth Society??
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Old 11-02-2013, 10:35   #18
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I should clarify we are discussing sailing into the wind not a down wind run.

Tom
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Old 11-02-2013, 10:37   #19
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Re: Settle an argument

Never argue with the FIL, even when he's wrong as in this case!
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Old 11-02-2013, 10:53   #20
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Re: Settle an argument

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Ok, had a friendly argument with my father in law this weekend. Last time we rented a sailboat for a day the instructor commented on the importance of sail trimming due to the fact the sail is basically a wing. Later on that day my father in law brings up that he believes this is all mumbo jumbo and that the "wing effect" is basically oversold. He claims that a perfectly workable sail could be made out of a flat solid sheet of material with no curve and you would get decent performance out of it. Not as good a a curved sail, but almost as good. I told him it might work but would basically be crap and give little to no forward thrust. Who is right?

Tom
If a curved wing surface is required to make lift in one direction explain to me how planes fly upside down.

Arvel Gentry, Boeing Engineer, sailor, using a flat plate to explain how lift is generated.

The Origins of Lift

Proper wing shape important, yes. Proper trim important, yes. But we're underpowered half the time due to lack of wind. Also, trying to sail upwind most of the force of the sail is pulling us in a direction we don't want to go, so doing everything possible to maximize the lift to drag ratio makes a big difference in how well you go to weather. You would still go to weather with a flat plate, just slower.
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Old 11-02-2013, 11:09   #21
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Re: Settle an argument

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Sailing dead downwind, FIL is 70% right, a cupped shape will provide more drive downwind than a flat plate.
Not if their areas are equal.

b.
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Old 11-02-2013, 11:12   #22
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Re: Settle an argument

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Not if their areas are equal.

b.
Even with equal areas and the same projected shape the flat plate will have less drive than the cupped plate.
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Old 11-02-2013, 11:19   #23
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Re: Settle an argument

F-104 Starfighter.
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Old 11-02-2013, 11:22   #24
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Re: Settle an argument

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F-104 Starfighter.
Huh?!?
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Old 11-02-2013, 11:23   #25
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Re: Settle an argument

Downwind, with a perfectly flat sail, airflow would just burble around the edges. With some curve, you will get a small amount of laminar airflow. The more, the more. Look at a spinnaker as an example.

Want to test it? Make a plywood sail, with 2x4 strongbacks to keep it flat. The boat will sail but in a pretty sucky fashion. Performance will be abyssmally underwhelming at any point of sail, worse in some than others but always awful.

Wing, schming. Don't think wing-shaped. Think SAIL-shaped. But it still needs shape.
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Old 11-02-2013, 11:24   #26
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Re: Settle an argument

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Not if their areas are equal.

b.
A thin disk has a Drag Coefficient of 1.1 vs a hollow semi-sphere cupping the wind which has a Drag Coefficient of 1.42. It is reasonable to assume that a slightly cupped disk has a Cd greater than 1.1 and less than 1.42.

Drag Coefficient
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Old 11-02-2013, 11:27   #27
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Re: Settle an argument

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F-104 Starfighter.
That's cheating. It's almost a rocket with a pilot and control surfaces strapped on. I remember that they had covers for the knife edge leading edge to not hurt people.
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Old 11-02-2013, 11:35   #28
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Re: Settle an argument

Homework time:

Lift (force) - Wikipedia, the free encyclopedia
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Old 11-02-2013, 11:36   #29
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Re: Settle an argument

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Even with equal areas and the same projected shape the flat plate will have less drive than the cupped plate.
Proof?

b.
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Old 11-02-2013, 11:40   #30
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Re: Settle an argument

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Proof?

b.
Two posts further along: Settle an argument
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